1. Field of the Invention
The present invention relates to novel borate compounds and their use as polymerization catalysts which are particularly effective for the polymerization of polyphosphazene synthetic rubbers.
2. Description of the Prior Art
Phosphazenes are compounds of the general formula A.sub.3 P.dbd.NB, wherein A and B may be organic radicals, hydroxyl, halides, or pseudohalides. Oligomers of phosphazenes may be linear or cyclic. At temperatures above 250.degree. C., the cyclic chlorophosphazenes polymerize to form long chain polymers of the formula (NPCL.sub.2).sub.n, a process first demonstrated by Schmitz-DuMont (ref. 196) and illustrated in U.S. Pat. No. 3,370,020 to Allcock. Polymers formed in this matter have many desirable characteristics, including low temperature flexibility, thermal stability, resistance to hydrolysis in corrosive atmospheres and very low flammability. Unfortunately, the production of polyphosphazene polymers by this route has been found to be expensive and the final product characteristics are not easily controlled.
Improved processes for polyphosphazene synthesis include the use of solvents such as isopropyl alcohol as described in U.S. Pat. No. 3,459,838 to Klender and U.K. Pat. No. 1,497,264 to Hudson et al.
Another improvement is the use of acid acceptor substances, primarily amines, and particularly pyridine which not only reacts with the liberated acid but may serve as a catalyst as well. The use of acid acceptors is described in the following representative patents: U.S. Pat. No. 3,468,981 to Bezman, U.S. Pat. No. 3,524,907 to Stockel et al., and U.S. Pat. No. 3,974,242 to Linear.
Lewis acid catalysts have also been described as being effective, as evidenced by U.S. Pat. No. 4,005,171 to Reynard et al. and U.S. Pat. No. 4,116,891 to Dieck. Organo-metallic catalysts have also been reported and are discussed in the Dieck et al. patent supra. and U.S. Pat. No. 4,123,503 to Snyder et al.
It has been reported that for the polymerization of hexachlorocyclotriphosphazene, at least a trace amount of water must be present (U.S. Pat. No. 3,937,790 to Allcock et al.) although greater amounts hinder the reaction and the presence of water may be required for the polymerization of other halogenated cyclic phosphazenes.
Organo-metallic compounds containing boron with at least one halogen substituent have been reported to be effective as a polymerization catalyst by Snyder in U.S. Pat. No. 4,123,503 and Fieldhouse and Graves in Phosphorus Chemistry, American Chemical Society, Washington, D.C. 1981. Such compounds have been reported to be particularly effective in reducing the time of completion of the polymerization process with higher yields than those reported by other processes. There exists however a need for better catalysts allowing both control of the percent conversion and degree of cross-linking, the formation of structures which can be further modified by curing and other post polymerization reactions, and which can be used conveniently in solvents so as to eliminate the necessity found in some systems for removing gas from the reaction mixture under vacuum.